In closed loop control systems which control the operation of air/fuel forming devices of internal combustion engines such as carburetor and fuel injection systems it is usual to employ a gas sensor to sense a component of the exhaust gases issued from the engine which is indicative of air/fuel ratio of the combustible mixture being fed therein. In most cases the sensor is an oxygen sensor which uses a solid electrolyte such as zirconium.
Although the above-mentioned zirconium type oxygen sensor functions satisfactorily when the gas sensor is relatively new, when deteriorated with age the gas sensor may produce an output signal which does not correctly indicate the instantaneous air/fuel ratio. When the gas sensor produces such a signal the air/fuel ratio is controlled away from the stoichiometric air/fuel ratio.
In order to prevent the above-mentioned undesirable feedback control the present applicant has proposed two methods in Japanese patent application No. 50-117244 and in Japanese utility model application No. 50-145316. One of them involves the variation of the reference signal in accordance with the variation of the maximum value of the output signal of the gas sensor and the other method involves the variation of the reference signal in accordance with the mean value of the output signal of the gas sensor. However, since the former method does not include any means for compensating the reference signal in accordance with the minimum value, the reference signal is apt to include a large error in a range where the variation of the minimum value is large. In the latter method, the reference signal is apt to vary undesirably as time advances because the reference signal is produced by averaging the maximum and minimum values in time.